The secret I think, is in intentionally doing better work now than you did an hour ago, no longer just yesterday. It’s easy, almost natural, to keep doing the same thing, the same effect, the same format, use the same fonts, etc. that you can do it in your sleep. But it’s actually just as easy to improve your work now at a faster rate. We can find instruction and inspiration within seconds and designers learn fast.
Don’t wait until you have the time to experiment on a personal project or land a more interesting one. Just do something new, different and better with whatever project you’re working on now. Anyway, in design, even the tiniest details can make a big difference.